Nowadays, there are applications using one key to perform two functions. For example, the power key on some cell phones can perform both power on/off and another function, e.g. an ‘exit’ function. When the cell phone is in the power-on mode, if a user presses the power key for less than a certain pre-defined time threshold, the key will trigger the ‘exit’ function. And if the user presses the power key for a longer period than the pre-defined time threshold, he will turn off the cell phone.
Traditionally a way to realize these multiple functions on one key is to use a power management chip or microcontroller or microcomputer. However, this traditional circuit is complex and expensive.
U.S. Pat. No. 5,140,178 describes a double function key especially a reset circuit for microcomputer.